Bring a large pot of salted water to a rolling boil. Add the pasta and cook according to the package instructions until al dente, then drain and rinse under cold water.

If using fresh shrimp, bring a small saucepan of water to boil. Add the shrimp and cook for 2-3 minutes until pink and opaque. Let cool, then chop. If using pre-cooked shrimp, chop them.

Dice the celery, red bell pepper, and red onion into small pieces. Soak the red onion in cold water for a milder flavor if desired. Finely chop the parsley.

In a small bowl, combine mayonnaise, sour cream, Dijon mustard, lemon juice, Old Bay seasoning, salt, and pepper, and whisk until smooth.

In a large bowl, combine the cooled pasta, chopped shrimp, imitation crab, and diced vegetables. Pour the dressing over and toss to coat.

Cover and refrigerate for at least 1 hour.

Before serving, gently toss again and garnish with parsley and paprika.